This invention relates to the protection of surfaces from excessive oxidation using an iridium-aluminum protective coating and, more particularly, to the prevention of excessive oxidation of the protected surface.
In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust-gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150xc2x0 F.
Many approaches have been used to increase the operating temperature limits of turbine blades, turbine vanes, and other hot-section components to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures. Physical cooling techniques may also be used.
The surfaces of the articles may be protected with an aluminum-containing protective coating, whose surface oxidizes to an aluminum oxide scale that inhibits further oxidation of the surfaces. However, the aluminum oxide scale is relatively permeable to oxygen. During service, oxygen diffuses from the environment and through the aluminum oxide scale to the underlying aluminum-containing protective coating, whereupon more aluminum oxide is formed, and to the substrate. The formation of too much aluminum oxide may lead to spallation of the aluminum oxide scale, consumption of the aluminum in the aluminum-containing protective coating, and the loss of protection of the underlying substrate. The additional oxidation of the substrate leads to dimensional changes and loss of strength in the substrate.
There is therefore a need for an improved approach to the aluminum-containing protective coatings on surfaces of materials used at high temperatures, such as nickel-base superalloys. The present invention fulfills this need, and further provides related advantages.
The present invention provides an aluminum-containing protective coating that is modified to have improved resistance to oxygen penetration, and a method for its preparation. The present protective coating may be used as an environmental coating or as a bond coat for an overlying ceramic thermal barrier coating.
A protected article comprises a substrate, preferably a nickel-base alloy such as a nickel-base superalloy, and a protective structure overlying a surface of the substrate. The protective structure comprises an iridium-aluminum protective coating overlying the surface of the substrate. A ceramic thermal barrier coating may overlie the protective coating.
The protective coating preferably comprises at least about 70 percent by weight iridium, and most preferably comprises from about 70 to about 90 percent by weight iridium. The protective coating preferably has a thickness of from about 10 micrometers to about 125 micrometers.
A method of protecting an article comprises the steps of providing a substrate including at least a portion of the article, and depositing a protective structure overlying a surface of the substrate. The protective structure comprises an iridium-aluminum protective coating overlying the surface of the substrate. Preferably, the iridium-aluminum protective coating is deposited by depositing a layer comprising iridium overlying the surface of the substrate, depositing a layer comprising aluminum overlying the layer comprising iridium, and heating the substrate, the layer comprising iridium, and the layer comprising aluminum to form the iridium-aluminum protective coating. The protective coating also typically includes elements diffused into the protective coating from the substrate, such as nickel, chromium, and the like, and may additionally be doped with modifying elements.
In this layered system, the iridium-aluminum protective coating oxidizes to form an aluminum oxide scale that protects the substrate article from excessively rapid oxidation. The iridium-aluminum protective layer itself also serves as an oxygen barrier layer in service to protect the substrate against excessive oxidation. The iridium-aluminum protective layer, because of the low permeability of oxygen in high-iridium alloys, inhibits the diffusion of oxygen to the underlying substrate. The use of an iridium-aluminum alloy, rather than pure iridium, prevents the iridium from vaporizing at elevated temperatures.